Question: Solve for $r$ : $-18 + r = -15$
Answer: Add $18$ to both sides: $ \begin{eqnarray} \\ -18 + r &=& -15 \\ \\ {+18} && {+18} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-18 + r} &\hphantom{=}& \hphantom{-15} \\ r &=& -15 {+ 18} \\ \end{eqnarray} $ Simplify. $ r = 3$